Examining pelagic carbonate-rich sediments as an archive for authigenic uranium and molybdenum isotopes using reductive cleaning and leaching experiments

Novel metal isotope systematics are increasingly used to understand environmental change in geological history. On a global scale, the isotopic budgets of these metals respond to a range of environmental processes, allowing them to trace complex changes in the global climate system and carbon cycle. In particular, uranium (U) and molybdenum (Mo) isotopes are useful tools for quantifying the global extent of oceanic anoxia and euxinia respectively. The oceanic signature of these metals is recorded in contemporaneous marine sediments. Whilst, traditionally, organic-rich anoxic ‘black shales’ have provided a useful archive of these metals, carbonate sediments are increasingly being used as a passive recorder of ocean chemistry. The majority of published U and Mo isotope studies come from shallow water platform environments. By contrast, pelagic carbonate sediments are an under-explored archive for these metals, yet are widely available for important periods of Earth history. Despite their advantages, carbonates are a complex archive, containing multiple ‘contaminant’ components such as Mn-oxides, organic matter and detrital minerals. Each of these phases can have different metal concentrations and isotopic signatures, giving the potential to distort or bias the true oceanic signature recorded by the carbonate. Reductive cleaning procedures and selective leaching protocols can be used to avoid these contaminant phases, and are tested here on modern and ancient samples to judge their efficacy in isolating a ‘carbonate-bound fraction’. To this end, leaching experiments were performed using different concentration acetic acid, HCl and HNO3, on reductively cleaned and uncleaned sample pairs. The data demonstrate that Mn-oxide coatings and exchangeable phases have a large impact on the Mo isotopic signature (δ98Mo) of carbonates, even when weak leaching techniques are used to preferentially dissolve them. Furthermore, detrital sources of Mo are also easy to liberate with different leaching protocols, and exert a significant control on leachate isotopic composition. The leaching studies identify that the pelagic carbonate end-member has a relatively high δ98Mo, but the precise relationship to seawater compositions remains unclear. For U, significant contributions from non‑carbonate phases can clearly be identified in higher concentration leaching acids using U/Ca ratios. However, U isotopes (δ238U) show no resolvable difference with different leaching procedures and are not affected by reductive cleaning. This result probably reflects (a) the low potential for leaching refractory residual detrital U phases (e.g., zircon) that contain the majority of U in the sample and (b) the low U inventories of Mn oxides versus those of Mo. Instead, leaching likely extracts U that is mineralogically bound in carbonates and authigenic clays, which share a common isotopic signature. These new data suggest that U incorporation into pelagic carbonates may be dominated by adsorption, and be offset from seawater by ~−0.15‰, in a similar manner to that seen for clays

The meaning of carbonate Zn isotope records: Constraints from a detailed geochemical and isotope study of bulk deep-sea carbonates

Studies of the zinc isotope (δ66Zn) composition of the past ocean often exploit the bulk carbonate sediment as an archive. The relationship of bulk carbonate δ66Zn to seawater values, however, is poorly understood. Experimental results suggest that inorganic carbonate records δ66Zn that is substantially higher than the aqueous Zn2+ pool. On the other hand, biogenic carbonate from modern cold-water corals and benthic foraminifera appear to record the ambient seawater δ66Zn value, without fractionation. Here, we address this problem through a detailed study of the different geochemical and isotope reservoirs in deep-sea carbonate samples. Carbonate-rich, pelagic sediment samples were used in this study, one of Holocene age and one from the mid-Cretaceous. The samples were subjected to extensive leaching experiments and interpreted using a mixing model to identify silicate and Fe-Mn oxide contaminants as well as pure carbonate phases. The chemical and isotopic characteristics of the contaminant phases are consistent with previous analyses of silicates and Fe-Mn oxides. The δ66Zn values of the pure carbonate end-members, identified using a mixing model, at +0.93 to +1.10‰, are within analytical uncertainty of results obtained via leaching with buffered 1 M acetic acid (pH 5). By contrast, the δ66Zn of un-buffered acetic acid leachates are up to 0.35‰ beneath these values, reflecting the influence of contaminant phases. Weak mineral acids (e.g. 0.2–0.5 M HCl) are lighter still, at up to 0.5‰ below carbonate end-members. The bulk carbonate sediment end-member δ66Zn for the Holocene sample are ∼0.37–0.54‰ above deep ocean values, consistent with the experimentally determined isotope offset between inorganic carbonate precipitates and an aqueous fluid. These carbonate end-members do not appear to directly record seawater compositions, unlike some biogenic carbonates. These differences identify the potential for ancient sediment δ66Zn records to reflect variations in Zn incorporation mechanism into the carbonate, rather than the δ66Zn of contemporary seawater, and hence may not reflect changes in Zn sources and sinks driven by environmental change. Further research is required to better identify the controls of δ66Zn in bulk carbonate sediments to reliably use them as an archive for seawater.ISSN:0016-7037ISSN:1872-953

Noble gases in cluster chondrite clasts and their host breccias

We measured noble gases in “cluster chondrite clasts” from nine unequilibrated ordinary chondrites (UOCs). For five meteorites, we also present data for so-called “clastic matrix,” the impact-brecciated material in which the angular to subrounded cluster chondrite clasts are often embedded. Cluster chondrite clasts are characterized by close-fit texture of deformed and indented chondrules with lower amounts of fine-grained interchondrule matrix than in other UOCs (Metzler 2012). They are ubiquitous in UOCs and may indicate accretion and compaction of hot and deformable chondrules within hours or days after formation. Clastic matrix of four of the five meteorites contains He and Ne implanted by the solar wind (SW), indicating that they are regolith breccias. In contrast, cluster chondrite clasts are essentially devoid of SW, confirming that they are fragments of “primary accretionary rocks” (Metzler 2012). Trapped Kr and Xe in all samples are essentially primordial (type “Q”). Trapped Xe concentrations in cluster chondrite clasts are similar to values in other UOCs of similar metamorphic grade despite their low fractions of primordial gas-bearing fine-grained materials. This possibly indicates that the interchondrule matrix in cluster chondrite clasts is more pristine than matrix of regular UOCs. Later loss of primordial gases during parent body metamorphism is mirrored in the decreasing concentrations of primordial noble gases with increasing petrologic type. Relative to cluster chondrite lithologies, clastic matrix often contains excesses of cosmogenic noble gases, most likely due to precompaction exposure in the parent body regolith.ISSN:1086-9379ISSN:0026-1114ISSN:1945-510